There exists throughout the world major deposits of heavy oils which, until recently, have been substantially ignored as sources of petroleum since the oils contained therein were not recoverable using ordinary production techniques. For example, it was not until the 1980's that much interest was shown in the heavy oil deposits of the Alberta province in Canada even though many deposits are close to the surface and represent an estimated petroleum resource upwares of many billion barrels.
It is well known that heat can be employed to recover hydrocarbons from underground formations through wells, drilled in the underground petroleum deposits. Various methods have been developed over the years for primary and secondary recovery of oil from underground formations by thermal means.
Moreover, it is well-recognized by persons skilled in the art of recovering oil or petroleum from subterranean deposits that only a small fraction of the viscous petroleum may be recovered from subterranean formations by conventional, primary and secondary means. Some method, such as a thermal recovery process or other treatment, must often be applied to the formation to reduce the viscosity of the petroleum to a level where it will readily flow to wells from which it can be recovered to the surface of the earth. Steam and/or hot water flooding are commonly used for this purpose and have been very successful in some formations for stimulating recovery of viscous petroleum which is otherwise essentially unrecoverable. Steam flooding is a thermal oil recovery method which has enjoyed increased popularity in recent years and is often the most commercially practical method or process.
Steam flooding can be utilized in a single well by the so called "huff-and-puff" technique. That method involves first injecting steam into a vertical well, then shutting in the well for a "soak", wherein the heat contained in the steam raises the temperature and lowers the viscosity of the petroleum. Thereafter, a production period begins wherein mobilized petroleum is produced from the well, usually by pumping. This process can be repeated over and over again.
Steam flooding may also be utilized as a stem or thermal drive means or a steam through-put process, wherein stem is injected into the reservoir through one or more vertical injection wells. This steam then moves through the subterranean reservoir mobilizing and volatilizing the petroleum it encounters. This steam-flood front moves through the reservoir towards a production well wherefrom the petroleum fluids are produced. This steam drive process is often more effective than the "huff-and-puff" method inasmuch as the potential volume of the reservoir which can be swept by the process is greater.
Although the steam drive process is very effective in recovering petroleum from the portions of the reservoir through which the steam sweeps, in practice, the success of the stem drive method is often poorer because the processes inability to develop liquid communication and because of low vertical conformance efficiency. It is typical that less than 50% of a petroleum contained within a formation can be recovered by the steam drive process thereby leaving large amounts of petroleum within the reservoir after the completion of the process.
Another common thermal recovery technique is the "cyclic steam stimulation" method, wherein production of oil from a well is periodically interrupted and steam is injected into the well. The steam supplies heat to reduce the viscosity of the oil remaining in the oil-bearing strata surrounding the well so that it will flow more readily to the well for production therefrom. The cycle is repeated until the production index becomes smaller than a minimum profitable level. Sometimes the term cyclic steam stimulation and huff-and-puff are used interchangeably.
One of the problems faced with a thermal oil recovery method arises from the varying permeabilities of the reservoir. Where there is a permeable zone with a considerable increase in permeability when compared to the oil-bearing strata, the injected steam will flow into the permeable zone preferentially, or, on occasion, almost exclusively. Another problem encountered is the loss of a portion of the heat already transferred to the oil-bearing strata by the stem as a result of conduction way into the permeable zone. Clearly improvements are needed.